1. Field of the Invention
The present invention relates to a process for the preparation of alkyl and aromatic ketones, aldehydes, or carboxylic acids. More preferably, the invention relates to a process for preparing aromatic ketones.
2. Description of the Related Art
Aromatic ketones are important synthetic intermediates for a number of valuable chemical compounds such as herbicides. Several such compounds are isoxazole and derivatives thereof (Cain et al., Euro. Pat. App. EP 470856 (1992)). These types of ketones are normally prepared by the Friedel-Crafts acylation of an aromatic nucleus. However, in some cases this methodology does not produce the desired isomer, and more indirect methods must be employed.
Conversion of nitro groups to the corresponding carbonyl can be achieved under Nef reaction conditions known in the art. For example, the preparation of aromatic ketones by an oxidative Nef reaction has been previously reported. Such aromatic ketones are shown to be useful as synthetic intermediates in the preparation of chemical pharmaceuticals. (Reid et al., Tetrahedron Lett. 31:1093 (1990)). Classic Nef reactions are performed by adding a solution of a nitronate anion to a strongly acidic aqueous solution. However, these conditions achieve only minimal conversion of the nitro group of a nitronate anion substituted on an aromatic ring to the corresponding ketone. Conversion to the desired ketone can be achieved under oxidative Nef reaction conditions. However, a large excess of hydrogen peroxide, which is economically unattractive and potentially hazardous, is necessary. (Olah et al., Synthesis 662 (1980)).
Other means by which an oxidative Nef reaction may be achieved include the use of potassium permanganate, ammonium/sodium persulfate, or t-butylhydroperoxide with VO(AcAc).sub.2. However each of these suffers from several drawbacks. Potassium permanganate has low solubility and its use produces large amounts of manganese dioxide as a by-product which makes product isolation difficult. (Reid et al., Tertahedron Lett. 31:1093 (1990); Schechter et al., J. Org. Chem. 27:3699 (1962)). An oxidative reaction using ammonium/sodium persulfate not only requires a large amount of the reagent but is quite sluggish and does not go to completion. (Pagano et al., J. Org. Chem., 35:295 (1970)). The use of t-butylhydroperoxide with VO(AcAc).sub.2 gives very poor yields. (Bartlett et al., Tertahedron Lett. 4:331 (1977)).
Accordingly, there exists a need in the art for an efficient and cost-effective process for preparing aromatic ketones.